FLOOD07489

• County: Statewide
• Basin: Statewide
• Title: Appeals, Revisions, and Amendments to Flood Insurance Maps - A Guide for Community Officials
• Date: 1/1/1990
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

For riverine, lacustrine, and coastal
flooding sources studied by detailed
methods, FEMA performs analyses of the
100-year flood and, usually, the 10-, SO-,
and SOO-year floods. Often a hydraulic
analysis of the 100-year floodway is
performed for riverine flooding sources.
For sheetflow and alluvial fan flooding
sources studied by detailed methods,
and for all flooding sources studied by
approximate methods, FEMA usually
performs analyses of only the 100-year
flood.

Therefore, the extent of the hydrologic
and hydraulic analyses that a revision
requestor may be required to submit is
determined not only by the basis of the
revision request, but also by the type of
flooding source and the scope of the
study on which the effective NFIP map is
based.

. Unless the revision request is based on
the use of alternative models or
methodologies, the hydrologic and
hydraulic analyses that the revision
requestor submits must be performed
with the models used in the preparation
of the effective NFIP map. For riverine
flooding sources studied by detailed
methods, the hydrologic analyses on
which NFIP maps are based are usually
performed with standard engineering
methodologies, such as flood-frequency
analyses of stream gage data, or with
widely accepted computer models, such
as the COE HEC-1 model or the SCS TR-20
model.

The hydraulic analyses for riverine
flooding sources are usually performed
with the COE HEC-2 step-backwater
model or a similar and widely accepted
model, such as the SCS WSP-2 model or
the USGS WSPRO model.

For the analysis of alluvial fan flood
hazards and the hazards associated with
coastal storm surge and wave action,
including wave height and wave runup,
FEMA has established or adopted special
methodologies and computer models.

For detailed analyses of lacustrine and
sheetflow flood hazards, and for
approximate analyses of all types of
flooding sources, FEMA uses a variety of
standard engineering models and
methodologies.

The revision requestor may ask FEMA for
copies of the input and output data from
the model(s) used, or copies of other
calculations or analyses performed, in
the preparation of a specific NFIP map.
(See Appendix C.)

. As required by Paragraph 6S.6(a)(6) of
the NFIP regulations, when a revision
request is based on the use of an
alternative hydrologic or hydraulic
model, the requestor must show that the
model used has been reviewed and
approved for general use by a Federal
agency responsible for water resources
activities or a notable scientific body, is
well documented (with a user's manual
that includes source codes), and is
available to the general user (i.e., in the
public domain and non-proprietary).

. When the requestor is required to
submit a new model, all differences
between the conditions represented by
the original and revised models must be
justified by physical changes that have
occurred, or the use of better hydraulic
data or revised discharges. The new
model should also account for all
encroachments that have occurred in the
floodplain since the original model was
developed.

. When a hydrologic or hydraulic analysis
is submitted to FEMA, copies of the input
and output data from the original and
revised computer models must be
submitted.

. For information concerni ng additional
data that must be submitted in support
of requests that involve changes to
floodways, refer to Chapter 7.

. When a request for a revision to
floodplai n boundaries is based on data
that show changed topographic

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